DISEASE RESISTANCE IN ANNUAL CROPS 57 



because of their survival value. They have survival value in host popu- 

 lations with differential genes for resistance, but there is ample 

 evidence that they have a selective disadvantage in the absence of 

 differential host resistance. 



These considerations led van der Plank (1968) to his theorem of 

 "homeostasis" diagrammed below: 



HOMEOSTASIS 



Susceptible hosts Resistant hosts 



Directional selection 



(Selection for virulence) 



Stabilizing selection 



(Directional selection pressure removed) 



When the pathogen population is presented with differ- 

 entially resistant hosts instead of susceptible ones 3 

 directional selection for virulence will take place. The 

 reverse change is effectuated by stabilizing selection 

 when the differentially resistant hosts are removed. 

 From: van der Plank s 1968. 



Directional selection for differential virulence by differentially resis- 

 tant hosts is balanced by the stabilizing selection due to decreased 

 fitness associated with differential virulence. In typically biotrophic 

 pathogens like rust the directional selection can be strong; in pathogens 

 with an important non-parasitic phase stabilizing selection is strong. 



TOWARDS A STRATEGY 



PREMISES 



A strategy against blister rust (pathogen Cronartium ribicold) of 

 western white pine (Pinus monticola) , to concentrate on this particular 

 problem, has to be mapped out. The premises are: 



1. There is a fair degree of intraspecific variability in western white 

 pine (Ahlgren, 1968; Bingham, 1966). 



2. In the western white pine area there exists a great variety of environ- 

 mental conditions. 



3. Ribes species, serving as alternate hosts of the rust, cannot be 

 eradicated completely (Peterson and Jewell, 1968). 



